This invention relates to an improved process for converting wet acetylene-containing streams primarily to aromatics using a zinc-promoted, crystalline borosilicate molecular sieve catalyst composition and, more particularly, to an improved process for converting wet, impure, acetylene-containing streams to a product rich in aromatics, particularly benzene, toluene, and xylenes, using a promoted catalyst composition made by incorporating a major amount of a HAMS-1B crystalline borosilicate molecular sieve composited in an inorganic support with a minor amount of a zinc compound and calcining the result to form a promoted material containing supported zinc, essentially in the form of the oxide.
Methane (natural gas) is expected to become a significant feedstock for the production of fuels and chemicals importantly because of the large amounts that become available in crude oil production. Proven technology exists to convert methane by (1) methane pyrolysis to form ethylene-acetylene mixtures or primarily acetylene, and (2) partial oxidation to mixtures of gases containing 5-10 mol percent of acetylene. Other gases which may be present in the products of either the partial oxidation or pyrolysis technique are hydrogen, oxygen, nitrogen, water, carbon monoxide, carbon dioxide, methane, ethane, propane, and the like. For example, a typical output stream from a methane pyrolysis plant contains acetylene, hydrogen, methane, ethylene, carbon monoxide, carbon dioxide, nitrogen, and higher acetylenes. Catalysts which can effectively convert acetylene to useful liquid products, particularly aromatics, in the presence of these other gases are relatively few. The value of such catalysts lies in their ability to directly convert methane to hydrocarbon transportation fuels without going through oxygenated intermediates.
Zirconia-alumina was recently discovered to be such a catalyst. However, at vapor pressures of water of about half that of the acetylene or greater in the feed stream, the products obtained over zirconia-alumina contain a wide range of aliphatic and aromatic oxygenates. See U.S. Pat. No. 4,585,897.
Another catalyst which has been used is based on unsupported zeolites with a crystal framework structure similar to the crystalline aluminosilicates of the ZSM-5 family. High silica/alumina ratio crystalline aluminosilicates are preferred. See U.S. Pat. No. 4,424,401 and J. Catalysis (1983) 80, 207. While the latter publications give examples which employ the molecular sieve in the hydrogen form, H-ZSM, the sodium-exchanged form is also mentioned. Other catalysts suggested for acetylene conversion in these publications are crystalline aluminosilicates containing small amounts of Periodic Groups I-VIII metal ions in the crystal lattice. Several examples are shown in support of this latter claim, but they all employ an uncharacterized iron-containing ZSM-5 sieve. While three of the catalysts described give conversions of about 80 to 90% most of the other conversion results are much lower, less than 70% at 400.degree. C. It is also probable that the conversion generally drops off sharply with time as shown in Examples 19-20, with coking the probable cause of this loss of activity. Most importantly, the sieves employed are used unsupported.
Most catalysts for converting acetylene to aromatics are very sensitive to even trace amounts of water and oxygen. For example, U.S. Pat. No. 4,009,219 teaches that acetylene is converted to benzene in 99+% yield by a catalyst consisting of 0.2% potassium chromate on silica-alumina. This catalyst rapidly deactivates in the presence of water. V. O. Reikhsfeld and K. L. Makovetskii in Russian Chemical Reviews (1966) 35, 510-523 cite many examples of organometallic and Ziegler-Natta type catalysts which have also been employed, but these too decompose on exposure to air, water, or both.
Now it has been found that by incorporating a small amount of a zinc compound into a supported HAMS-1B crystalline, borosilicate molecular sieve and calcining, supported zinc-promoted catalyst compositions can be made which show considerably enhanced conversions of a wet acetylene-containing stream to aromatics, particularly benzene, toluene and xylenes, even in the presence of one or more impurities such as hydrogen, oxygen, nitrogen, carbon dioxide, carbon monoxide, organic oxygenates and hydrocarbons such as methane, etc. Such a process could provide the basis for the direct upgrading of such acetylene-containing streams to hydrocarbon products useful, for example, in the transportation fuels industry.